A static magnetic field is used by Magnetic Resonance Imaging (MRI) system to align the nuclear spins of atoms as part of the procedure for producing images within a subject. This static magnetic field is referred to as the B0 field or the main field.
Magnetic field gradient coils are used to generate spatially and temporally variying magnetic fields which are used to spatially encode the nuclear spins being imaged. This spatial encoding is part of what allows the reconstruction of images from magnetic resonance imaging signals.
However Magnetic Resoance Imaging is typically performed in a large magnetic field. As current flows through a magnetic field gradient coil the Lorentz force on the coil may be enormous. Gradient coils are typically mounted on or embedded within a rigid carrier to which these forces are transferred. During operation the large forces exerted by the Lorentz for can cause acoustic vibrations in the gradient coil and the rigid carrier. These acoustic vibrations may sound like a large knocking, thumping, or clicking sound during the use of the magnetic resonance imaging system.
It is commonly known that increasing the strength of the B0 field used for performing a magnetic resoance imaging scan offers the opportunity of increasing the spatial resolution and contrast resolution of the diagnostics images. This increase in resolution and contrast benefits physicians using a magnetic resoance image to diagnose a patient. However as the strength of the B0 field increases, so do the Lorentz forces acting on the gradient coil during use. As the B0 field increases so does the noise generated by the gradient coils during operation.
In the journal article Michael Poole, Richard Bowtell, Concepts in Magnetic Resonance Part B, Vol. 31B(3), page 162-175, 2007 a method of designing gradient coils using the boundary element is disclosed. The minimization of a functional during the design process which imposes torque-balancing of the gradient coil is disclosed.
In U.S. Pat. No. 5,764,059, an acoustically screened magnetic coil which is adapted to be placed in a static magnetic field is disclosed. Essentially a combination of active and magnetic screening for gradient coils is disclosed. A closed loop of the gradient coil carrying current is arranged such that the two different parts of the loop are mechanically coupled, dimensioned and arranged such that the Lorentz forces experienced by the magnetic equipment are substantially reduced and preferably cancelled. the US patent application US2004/0113618 shows a gradient coil system having two structurally independent sub-coils. These sub-coils are attached separately from one another so that an (RF) antenna system can be arranged between them.